1. Field of the Art
The present invention relates to a tool steel, particularly to a nitrogen containing powder metallurgical steel (hereinafter referred to as "PM steel" ), wherein the amounts of C, N and V are properly adjusted.
2. Description of Prior Art
It is known that the properties of tool steels containing alloying elements such as Cr, W, and V can be improved by incorporation of nitrogen into the steels (see, for example, Kobe Steel Technical Bulletin, R & D, Vol. 24 No. 3, pages 11 to 15, and Japanese Patent Application Laid-Open Specifications Nos. 78606/74, No. 49109/75 and No. 49156/75), and these steels are widely used as jig materials such as die materials and cutting tool materials because they have good wear resistance and good heat resistance. By nitriding treatment, a nitride of the type MX or M.sub.6 X (in which M stands for an alloying element and X stands for carbon or nitrogen) is formed, and this nitride is more stable than a carbide of the type MC or M.sub.6 C. Accordingly, the appropriate quenching temperature range is broadened and control of the heat treatment can be facilitated.
Further, the temper hardening characteristic is improved and a fine austenitic crystal structure can be obtained to improve the mechanical properties. Furthermore, the machinability of the steels can be improved.
According to Japanese Patent Publication No. 19774/1971, addition of 0.05 - 0.35% N to a die steel for high temperature service contributes to increase resistance to softening at high temperature, to suppress fatting of grain boundaries, and further contributes to suppress undue formation of delta ferrite.
Most conventional nitrogen containing tool steels have heretofore been prepared by a smelting process. When the smelting process is adopted for production of nitrogen containing tool steels, it is necessary to perform complicated steps such as the step of melting steel in a high pressure nitrogen atmosphere or the step of throwing a nitride into molten steel. Further, according to the smelting process, since the amount of nitrogen included in the steel is small and it is difficult to form a fine carbonitride and distribute it uniformly in steel, it is impossible to improve the properties to desirable levels.
As a means of overcoming the defects or limitations involved in the smelting process, methods have recently been proposed for obtaining nitrogen containing tool steels by the powder metallurgical process or the powder forging process. In those methods, by utilizing the fact that powder has a large specific surface area (surface area/volume) and the fact that a powder sintered body has a porous structure, an optional amount of nitrogen can be included in steel by a simple means, for example, by adding nitrogen in advance to the starting powder or adjusting the heating temperature, the heating time or the nitrogen partial pressure in the treatment atmosphere at the sintering step. It is expected that nitrogen will be fairly and uniformly distributed in steels according to these methods.
In conventional nitrogen containing PM steels, the machinability is not as highly improved as might be expected, as is apparent from Japanese Patent Publication No. 37810/1972. N is not a desirable element for stabilizing retained austenite when the steel is to be used for gauge. (see, for example, Japanese Patent Publication No. 9900/1972)
Rather, the machinability is degraded by incorporation of nitrogen into the steels. Accordingly, it is often said that the value of nitrogen containing high speed steels produced by powder metallurgical process is questionable. Moreover, several nitrogen containing high speed PM steels which have recently been put into practical use, have exhibited good machinability and good wear resistance in combination. The reason for this has not been elucidated. In particular, the relation between amounts of alloying elements which impart excellent machinability to steel and the amount of nitrogen enrichment is not clarified. Therefore, the kinds of steels which are enriched with nitrogen for the production of high speed PM steels and which are applicable are drastically limited. For example, Kobe Steel Technical Bulletin, R & D, Vol. 24, No. 3, page 10 discloses that when 0.4 - 0.5% N is added to Mo type high speed PM steels (JIS SKH 9 and modified JIS SKH 55), the machinability is remarkably improved.
Taking advantages of the fact that N addition is very advantageous for fine and uniform carbide, we carried out research on the improvement of various properties required for tool steel, particularly wear resistance and impact property with respect to various steel compositions. Then finally the present tool steel has been invented.